1. Preparation of Sulfide Powder (1) Mgs Powder Powder Of

Transactions of the Japan Institute of Metals, Vol. 25, No. 10 (1984), pp. 692 to 697

Ionic and Positive Hole Conductivities of Solid Magnesium and Strontium Sulfides*

By Hiroaki Nakamura**, Youichi Ogawa**, Koki Gunji† and Akira Kasahara**

The electrical conductivity of MgS and SrS disks carefully prepared to avoid any contamination has been measured at temperatures from 973 to 1223 K and in the Ps2 range from 10-10 to 104 Pa. Because the conductivity was independent of sulfur pressure in the low sulfer pressure range, it was concluded that MgS and SrS may be ionic conductors. The specific conductivity can be expressed as follows;

and the apparent activation energy for the conduction of MgS and SrS is 218 kJ/mol and 180 kJ/mol, respectively. However, in the high sulfur pressure range, the specific conductivity of both sulfides increases with an increase in sulfur pressure, suggesting the positive hole conduction. (ReceivedApril 23, 1984) Keywords: magnesium sulfide, strontium sulfide, electrical conductivity, sulfur pressure, ionic conduction, hole conduction, activation energy, alternating current bridge, polarization

and strontium were very carefully prepared . Introduction and served for the measurement of electrical conductivity in a wide range of sulfur partial Sulfides have been paid attention to as solid pressure. Potentiostatic polarization was also electrolyte. The physico-chemical properties of carried out to determine the mechanism of con- most of them have not well been studied, com- duction. pared with oxides and halides in actual use as electrolytes. Ⅱ. Experimental Method In this study the electrical conductivity of 1. Preparation of sulfide powder the sulfides of magnesium (MgS) and strontium (SrS), which are the same alkaline earths (1) MgS powder as calcium(1), was measured in order to seek for Powder of MgS was prepared by reacting the potential use of them as solid electrolytes. magnesium metal with sulfur. In a glove box in MgS is white gray, and it is known to have a which purified Ar gas flowed, a block of high- face centered cubic structure of NaCl type. Its purity magnesium (99.99%) was filed to melting point is said to be over 2273 K(2). In ad- powder, and it was mixed with an equal molar dition to the monosulfide, other polysulfides of amount of high-purity sulfur powder made strontium are known to exist: SrS2, SrS4 and by the high-temperature partial oxidation SrS5(3). SrS is white, and it has an fcc structure method(4). The mixture was immediately with a melting point over 2273 K(2). In the pre- vacuum sealed in a quartz capsule. This cap- sent investigation, monosulfides of magnesium sule was heated to 1020 K in more than 18×103ks to obtain MgS powder. The powder * This paper was originally published in Japanese in J . was white, and X-ray diffraction revealed no Japan Inst. Metals, 47 (1983), 21. impurities in it. ** National Research Institute for Metals , 3-12 2- (2) SrS powder Chome, Nakameguro, Meguro-ku, Tokyo 153, Powder of SrS was made in a manner similar apan. J † Present address: Central Research Laboratories to the preparation of CaS powder(1). SrSO4 of , Sumitomo Metal Industiries, Ltd., Amagasaki 660, reagent grade was reduced with purified Japan. hydrogen gas containing 1% H2S for 36ks at

Ⅰ Ionic and Positive Hole Conductivities of Solid Magnesium and Strontium Sulfides 693

1173 K. The powder of SrS thus produced was white, and X-ray diffraction showed no other lines than SrS. 2. Formation and sintering of sulfide disks

(1) MgS disks MgS powder was weighed in the purified Ar gas flowing in the glove box, and pressed with a 294 to 686 MPa pressure to form a disk of 1 to 4×10-4m2 area and 2 to 4mm thickness. The disk together with MgS powder was vacuum sealed in a quartz capsule, and it was sintered at 1470 K for 180ks or 360ks. X-ray diffraction showed no introduction of impurities nor oxidation of MgS disks. The density of a sintered disk was 2.37Mg/m3 for 180ks sintering and 2.40Mg/m3 for 360ks sintering. By comparison with the density of 2.66Mg/m3 calculated from the lattice constant, the porosity is 10.9% and 9.8%, respectively. The disks used for the conductivity measurement were those sintered for 360ks. (2) SrS disks The sintering of SrS disks followed the Fig. 1 Relation between specific conductivity of MgS and method for CaS(1). SrS disks imbedded in SrS sulfur pressure. powder were put in an SrS crucible which was placed in a MgO crucible, and they were measured conductivity of MgS and the partial sintered in a flow of He gas at 1800 K for 18ks. pressure of sulfur at a temperature of 973 to X-ray diffraction revealed no dissolution of im- 1223 K. It is found from the figure that the con- purities nor oxidation of the disks. The density ductivity is independent of the partial pressure of a sintered disk was 3.48Mg/m3, and the in the lowest range of Ps2. In combination with porosity was estimated from the lattice-cons- the polarization behavior given later in this tant density of 3.66Mg/m3 to be 4.92%. report, MgS is an essentially ionic conductor in this range of Ps2. The upper limit of Ps2 (10-6 3. Measurement of electrical conductivity to 10-7.5 Pa) at which the conductivity begins The method of measurement followed that to change depends on temperature. The lower for CaS(1). The partial pressure of sulfur was limit was unable to be detected. The conductivi- controlled by the mixing ratio of H2 gas to H2S ty in the essentially ionic region at T=1048 to gas, ranging from Ps2=10-10 to 104Pa. The 1223 K is expressed by AC bridge method with 1 kHz was applied to the conductivity measurement at 973 to 1223 (1) K. On the other hand, for Ps2>10-4 Pa the con- . Experimental Results ductivity changes in linear proportion to, and it can be expressed at T=1048 to 1223 K 1. Dependence of the electrical as conductivity on the sulfur partial pressure and temperature (1) Electrical conductivity of MgS (2) Figure 1 shows a relationship between the

Ⅲ 694 Hiroaki Nakamura, Youichi Ogawa, Koki Gunji and Akira Kasahara

This indicates that MgS is a p-type semi-con- by a predominant defect whose mobility is very ductor for Ps2>10-4 Pa. small. The temperature-independent conduc- (2) Electrical conductivity of SrS tivity may indicate that the equilibrium cons- Figure 2 shows a relationship between the tant for the defect reaction should be indepen- measured conductivity and the partial pressure dent of temperature. of sulfur at a temperature of 973 to 1223 K. 2. Potentiostatic polarization The conductivity is independent of the sulfur partial pressure for lower values of Ps2. The up- To determine whether the conduction in per limit of Ps2 in this range depends on MgS and SrS is electronic or ionic, poten- temperature. However, the lower limit of the tiostatic electrolysis was applied to these range was not detected. In this range SrS is con- materials. Typical data are shown in Figs. 3 sidered as an essentially ionic conductor, and and 4. In the region where the conductivity is the conductivity can be expressed at T=993 to independent of Ps2, the electrical resistance of 1223 K as both MgS and SrS increases with the time of electrolysis, a polarization taking place. The in- (3) crease is thought to be due to the precipitation of ions at the sulfide/electrode interface. For higher Ps2 the conductivity increases Therefore, MgS and SrS are considered to be with the rise of Ps2, an indication of p-type ionic conductors in the above region. semiconductor. In the p-type region, the con- In the region where the conductivity depends ductivity is independent of temperature. While on Ps2, no change of resistance was observed the time to reach equilibrium was as short as for potentiostatic electrolysis of MgS and SrS, 7.2ks in the region of essentially ionic conduc- an indication of electronic conductors. tion, it was over 36ks for p-type semiconduc- tors. No oxidation during a measurement was observed. The very slow equilibration for p- type semiconductor is presumed to be caused

Fig. 3 Relation of electrical resistance of MgS to time at 1223 K.

Fig. 2 Relation between specific conductivity of SrS and Fig. 4 Relation of electrical resistance of SrS to time at sulfur pressure. 1223 K. Ionic and Positive Hole Conductivities of Solid Magnesium and Strontium Sulfides 695

2. Electronic conduction in MgS and SrS Ⅳ. Discussion As seen fromFig.1, log σ changes linearly 1. Ionic conduction in MgS and SrS with log Ps2 for higher values of Ps2. No MgS and SrS have been found from Figs. 1 polarization by potentiostatic electrolysis was and 2 and the potentiostatic electrolysis to be observed in this range of Ps2. Therefore, a ionic conductors for lower values of Ps2. On single type of defects is considered to be the other hand, the range of Ps2 for ionic con- predominant in this range. duction in CaS(1) is much wider than those of MgS and SrS. Figure 5 shows a comparison of the dependence of conductivity on temperature between these sulfides in the region of ionic conduction. An inflection point exists at 1048 K for MgS and at 993 K for SrS. This seems due to lattice defects caused probably by impurities which are predominant at lower temperatures and in- significant compared with intrinsic defects at higher temperatures. Table 1 lists some physical properties (ion radius, lattice parameter, activation energy for electrical conduction and diffusion, etc.) of sulfides and oxides of Mg, Ca and Sr, all of which have an fcc structure of NaCl type. The activation energy (180 kJ/mol) for ionic conduction in SrS is very close to that for cations in the other materials, and hence the charge carrier in SrS is assumed to be Sr2+ ions. The activation energy (218 kJ/mol) for MgS, on the other hand, is smaller than that for anionic conduction in other materials, but it is not very close to that for cationic conduction. Therefore, Mg" is a possible candidate for the Fig. 5 Electrical conductivity of magnesium, calcium charge carrier, but further investigation is and strontium sulfides as a function of reciprocal necessary for a definite conclusion. temperature.

Table 1 Activation energy for electrical conduction and related data on sulfides and oxides.

*The apparent activation energy for diffusion . 696 Hiroaki Nakamura, Youichi Ogawa, Koki Gunji and Akira Kasahara

When doubly charged vacancy of magne- that of SrS made by reducing SrSO4 have been sium (V"Mg)is assumed to be predominant, measured at 973 to 1223 K for Ps2=10-10 to defect reaction can be written as 104 Pa. (2) The electrical conductivity of MgS is in- (4) dependent of Ps2 for Ps2<10-6～10-7.5 Pa, and MgS is assumed to be an essentially ionic where Ss denotes a sulfur atom at its normal lat- conductor in this range. The conductivity is tice and h•Ean electron hole. When the concen- proportional to for Ps2>10-4 Pa, and tration [h•E] is denoted by P, the equilibrium MgS is considered as a p-type semiconductor constant for eq. (4) can be written as with either vacancies of Mg cation or interstitial S anions as predominant defects. (5) (3) The electrical conductivity of SrS is independent of Ps2 for Ps2<10-2～10-4 Pa, The condition for electrical neutrality is such where SrS is an essentially ionic conductor. that 2[V"Mg]=p, and hence For Ps2>10-1 Pa, the conductivity increased with increasing Ps2, but the measurements were (6) unstable and unreliable. (4) The activation energy for ionic conduc- or tion in MgS and SrS has been determined and (7) compared with that for conductivity and diffusion in other alkaline-earth metal sulfides and oxides. As a result, the charge carrier in SrS On the other hand, if interstitial ions of has been assumed as Sr2+ ions. The activation sulfur (Si") are predominant defects, we have energy of MgS is rather large, and hence Mg2+ ions are not exclusively determined as (8) the charge carrier.

The equilibrium constant is given by Acknowledgments We would like to express our sincere thanks (9) to Professor K. Goto and Dr. K. Nagata of Tokyo Institute of Technology for their useful Since the electrical neutrality condition is comments. We also thank Mr. E. Momose and 2[Si"]=p, the last equation can be rearranged T. Tamura for experimental assistance. to REFERENCES (10) (1) H. Nakamura and K. Gunji: J. Japan Inst. Metals, 42 (1978),635 (in Japanese); Trans. JIM, 21 (1980),375. or (2) G. V. Samsonov and S. V. Drozdova: Sulfide, (11) Metallurgiya, Moscow, (1972). (3) H. D. Lutz: Z. anorg. and allg. Chem., 339 (1965), From eqs. (7) and (11), both defects are able 308. to account for the measured dependence of (4) H. Nakamura and K. Gunji: Japanese Pat. No. conductivity on Ps2. 1099459. (5) W. L. Worrell, V. B. Tare and F. J. Bruni: High The measured conductivity of SrS for higher Temperature Technology, Proceedings of the Third Ps2 has been unable to be explained by any International Symposium,held in Asilomar, Califor- defect models to be considered. nia 1967, International Union of Pure and Applied Chemistry, Butterworths, London, (1967),p. 503.

ⅴ. Conclusion (6) K. Nagata and K. S. Goto: Met. Trans., 5 (1974),899. (7) K. Ono, H. Ishihara and J. Moriyama: J. Japan Inst. Metals, 44 (1980), 185 (in Japanese). (1) The electrical conductivity of MgS made (8) B. C. Harding, D. M. Price and A. J. Mortlok: Phil. of Mg metal and sulfur of very high purity and Mag., 23 (1971), 399. Ionic and Positive Hole Conductivities of Solid Magnesium and Strontium Sulfides 697

(9) Y. Oihi and W. D. Kingery: J. Chem. Phys., 33 (11) Y. P. Gupta and L. J. Weirich: J. Phys. Chem. (1960), 905. Solids, 28 (1967), 811. (10) L. H. Rovner: Ph. D. Thesis, Cornell University, (12) T. Otowa, M. Kobayashi, K. S. Goto and M. (1966) (Technical Report No. 10, Contract No. Nonr- Someno: J. Japan Inst. Metals, 43 (1979), 1181 (in 40 (31), Office of Naval Research, Washington, D. Japanese). C.).